The 26 October 2013 paroxysm of Etna

Fig. 1. During the final phase of the paroxysmal eruptive episode at Etna's New Southeast Crater (NSEC; center) of 26 October 2013, there was a series of powerful explosions, one of which is shown in this photograph taken from Monte Zoccolaro, on the southeast flank of the volcano. In the foreground, a white gas plume is rising from a small lava flow that is descending through the deep breach formed by collapse during the 27 April 2013 paroxysm on the east flank of the NSEC cone. At right, a dense column of ash is seen rising from the Northeast Crater. Photo taken by Francesco Ciancitto, INGV-Osservatorio Etneo.

On the morning of 26 October 2013, a new episode of lava fountaining (paroxysm) took place at Etna's New Southeast Crater (NSEC), nearly exactly six months after the previous episode on 27 April 2013. During the phase of strongest activity, also the Northeast Crater (NEC) started to intermittently emit fine ash, producing impressive plumes of grayish-brown color (Fig. 1). Finally, also the Bocca Nuova (BN) produced sporadic explosions, one of which led to heavy fallout of lithig material (ash and blocks composed of old, mostly altered rock) onto the flank of the central cone, forming a ground-hugging cloud resembling a pyroclastic flow. The gas cloud charged with pyroclastic material (ash and lapilli) that rose from the NSEC rose to a height of several kilometers and was then blown by the wind toward southwest, leading to fallout of pyroclastic material onto population centers as far as the central portion of Sicily. Two lava flow-fields formed, a more extensive one toward south and southeast, and a smaller one toward northeast. At the southern base of the cone, the lava invaded the area of Torre del Filosofo, destroying two wooden huts used by the Etna mountain guides as shelter and bar, and interrupting the dirt road linking the southern and northern flanks of the volcano.

Prelude

The 26 October eruptive episode was preceded by several weeks of sporadic, and weak explosive activity at the NSEC, which started on the morning of 3 September 2013. In repeated sequences, minor ash emissions were observed, which were accompanied by incandescent ejections visible at night (Fig. 2); the last of this type of activity was observed during the night of 6-7 October 2013.

Fig. 2. Images recorded by the surveillance cameras of the INGV-Osservatorio Etneo on the Montagnola (EMOH, high-sensitivity, and EMOV), during the various episodes of minor explosive activity at the NSEC in September-October 2013. The last frame, of 25 October 2013, shows a rather weak glow at the NSEC at 04:06 GMT (=local time -2), the first sign of renewed eruptive activity after 19 days of quiescence, which then progressively intensified to culminate in the paroxysm on 26 October.

Fig. 3. The western part of the Bocca Nuova before (left) and after (right) the formation of a new pit crater, which took place on the afternoon of 5 September 2013. The first photo (of Boris Behncke, INGV-Osservatorio Etneo) shows a flat lava platform on the crater floor, which had filled a previous pit crater during the activity at the Bocca Nuova in January-February 2013; this platform has largely disappeared in the image at right (photographed by Biagio Ragonese, Etnean mountain guide).

On the afternoon of 5 September 2013, a strong seismic signal originating at the BN was probably related to the collapse of the lava platform that had filled a pre-existing crater depression (pit crater) during the eruptive activity of January-February 2013 (Fig. 3, at left); a strong but short-lived glow coming from the BN just before daybreak on 6 September was possibly generated by weak explosive activity or further collapse of still-hot and incandescent material. The 5 September collapse led to the formation of a new, circular pit crater (Fig. 3, right). During the ensuing weeks, weak explosion sounds were occasionally heard coming from the new pit, but there were no direct observations of eruptive activity.

During September and October 2013, conspicuous degassing was also observed at the Voragine; finally, loud bangs and roars were audible from deep within the conduit of the NEC, and sometimes these sounds were heard up to 1 km away from the crater.

A new, initially very weak, resumption of Strombolian activity at the NSEC was observed during the early morning hours of 25 October 2013 (see last frame in Fig. 2); this activity gradually intensified during the late forenoon producing small anomalies in the images recorded by the thermal surveillance cameras of the INGV-Osservatorio Etneo on the Montagnola (EMOT) and at Monte Cagliato (EMCT), located on the southern and southeastern flanks of Etna, respectively. The Strombolian activity progressively increased during the afternoon and was almost continuous in the evening, with explosions occurring every few seconds, which launched incandescent pyroclastic material up to 100 m above the crater rim.

Paroxysm

Around 02:00 GMT (=local time -2) on 26 October, the eruptive activity gradually assumed the character of lava fountaining from two vents within the crater. Images of the thermal surveillance cameras showed the formation of an eruption column, which rose several kilometers above the summit of the volcano, and which progressively became more sustained and charged with pyroclastic material (Fig. 4 and 5). During the same interval, lava started to overflow from the area of the "saddle" between the two cones of the Southeast Crater, feeding a lava flow that spilled down the southwestern slope of the NSEC cone, and then expanded at its base, forming various lobes that expanded toward southwest, south, and southeast.

Fig. 4. Lava fountain and formation of an eruption column seen from Tremestieri Etneo, 20 km south of Etna's summit, at 03:45 GMT on 26 October 2013. The lava overflowing from the "saddle" between the two cones of the Southeast Crater toward southwest is also visible. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

Fig. 5. Eruption column from the NSEC at 04:40 on 26 October 2013, seen from Zafferana Etnea. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

At 04:00 GMT, the collapse of a portion of the crater rim near the "saddle" allowed a significant increase in the volume of lava issuing from the crater. For the next two hours, the activity continued with slowly increasing intensity, but without major variations. Lava fountains rose from two vents within the NSEC, the more strongly active lying in the center of the crater, whereas the other lay closer to its southeastern rim. A third vent became active on the "saddle" at a time poorly constrained; from this vent a lava flow spilled south and after having traveled 150-200 toward south, united with the lava flow already active since the early morning.

At 06:21 GMT, a dense column of grayish-brown ash rose from the NEC and rapidly reached a height of about 1 km (Fig. 6). Images recorded by the thermal camera EMCT show that this material was hot, in particular during the first moments of the activity, but also during the ensuing hours, minor thermal anomalies were frequently recorded (Fig. 7). Contemporaneously with the first ash emissions from the NEC, degassing became more conspicuous also at the Voragine, and remained elevated through the forenoon. Ash emissions from the NEC repeatedly alternated with dense vapor emissions, but after 10:00 GMT they became virtually continuous.

Fig. 6. Lava fountaining at the NSEC (left) and ash emission from the Northeast Crater (NEC) at right, about 30 seconds after the start of activity at the NEC (06:21 GMT on 26 October 2013). This view is from the "Mareneve" road, about 2 km from the village of Fornazzo, on the eastern slope of Etna. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

Fig. 7. This sequence of images recorded by the thermal camera of the INGV-Osservatorio Etneo at Monte Cagliato (EMCT), on the southeastern flank of Etna, shows the start of ash emission from the NEC (seen at right in the upper two frames) at 06:21 GMT on 26 October 2013, and some of the minor thermal anomalies recorded during the activity in the following hours. The lava fountaining activity at the NSEC is visible in the left part of each of the images.

During the hours of most intense lava fountaining at the NSEC, explosive activity occurred also at the BN; these explosions intensified when activity at the NSEC had already started to diminish, and culminated in a particularly strong event at 10:12 GMT, which emitted almost exclusively old but high-temperature rock debris (Fig. 8). This explosion produced an eruptive cloud whose basal portion, which was heavily charged with pyroclastic (and lithic) material, fell back onto the southwestern flank of the central cone, generating a phenomenon similar to a pyroclastic flow that traveled to a distance of about 200 m from the crater rim and left a pink colored deposit containing numerous blocks of altered rock.

Fig. 8. Explosion at the Bocca Nuova (seen at left in these images recorded by the thermal surveillance camera of the INGV-Osservatorio Etneo on the Montagnola, EMOT), at 10:12 GMT on 26 October 2013. The two lower images show the collapse of the eruptive cloud, which created an avalanche of hot material similar to a pyroclastic flow. The lava fountain at the NSEC is seen in the right portion of each image.

At the NSEC, lava fountaining continued with slightly fluctuating intensity until about 10:00 GMT and then started to show a rapid diminution. During the final phases of lava fountaining, lava overflowed the southeastern crater rim and plunged into the deep scar produced during the collapse of that part of the cone during the 27 April 2013 paroxysm. This lava flow slowly advanced on top of the flow emitted through the same scar on 27 April, and eventually reached a length of 1.3 km. During this time interval, activity at the NSEC took on the character of Strombolian explosions, often rather violent and launching large pyroclastics mostly onto the northeastern portion of the cone, and generating loud detonations and air concussions. Strombolian activity continued at gradually diminishing intensity until about 13:00 GMT.

After the cessation of eruptive activity at the NSEC, ash emission continued from the NEC; at 17:27 GMT activity resumed at the NSEC with a series of explosions and ejection of large incandescent pyroclastics (Fig. 9), but after 18:15 GMT this activity diminished again, and small explosions continued until the late evening. Ash emissions from the NEC also ceased on the late evening.

Fig. 9. Resumption of explosive activity at the NSEC on the evening of 26 October 2013, recorded by the high-sensitivity visual surveillance camera of the INGV-Osservatorio Etneo on the Montagnola (EMOH).

The lava flow-field formed on the southern side of the NSEC continued to expand slowly on 27 October, and after having invaded the "parking lot" of Torre del Filosofo (the point of arrival of the 4WD tourist buses and the two wooden huts used by the Etnean mountain guides), it split into three main branches, the westernmost of which stagnated at the northern base of Monte Frumento Supino. A second branch had interrupted the dirt road leading to Torre del Filosofo, passing near the northeastern flank of the 2002-2003 cone, whereas the third lobe expanded into the direction of the former "Belvedere" monitoring station, stopping short of the western rim of the Valle del Bove. The map of the lava flows emitted during the 26 October 2013 paroxysm is shown in Fig. 10. The total volume of lava emitted in this event is about 1 x 10⁶ m³. A more detailed report authored by De Beni and Behncke on the mapping of the lava flows and the NSEC cone is available (in Italian) clicking on this link.

Fig. 10. Map of lava flows emitted from the NSEC on 26 October 2013; NSEC cone updated as of September 2013 (base DEM is of August 2007). BN = Bocca Nuova; SEC = Southeast Crater; NSEC= New Southeast Crater. Mapping carried out by the Laboratory of Cartography of the INGV-Osservatorio Etneo.

Airborne pyroclastic material of the NSEC paroxysm on 26 October 2013 was distributed toward southwest, affecting a rather narrow sector around the town of Adrano, but sparing towns adjacent to the north (Bronte) and south (Biancavilla). At Adrano, the deposit consists of coarse-grained ash, whereas in more distant locations, such as Caltanissetta, Centuripe, Enna and Montedoro, the material is more fine-grained. In the summit area of Etna, the deposit consists of lapilli a few centimeters in diameter, and only very few bombs (up to 20 cm in diameter) were found at the southern base of the cone. Large bombs are more numerous at the northern base of the cone.

The material emitted from the NEC on 26 October 2013 consists of a rather fine-grained fraction (the clasts being around 0.5 mm across) characterized almost exclusively of lithic material of various nature (reddish altered lavas, black volcanic fragments, secondary alteration minerals of white to pink color), and tachylitic clasts, as well as fresh sideromelane particles of major dimensions (around 2-4 mm), with irregular, elongate morphologies from fluidal to convolute, of dark tan color (information cited from the report by Andronico, Cristaldi and Lo Castro on the distribution of air-fall deposits and characteristics of the erupted products).

In its main outlines (such as lava fountain and eruption column heights and lava volume), the 26 October 2013 eruptive episode has been similar to many of the preceding episodes; in terms of eruptive intensity, it has been less violent than some of the paroxysms of February-April 2013 and other paroxysms in the past. However, the contemporaneous explosive activity at the NEC and the BN represents a particular element - the last time that the NEC showed significant eruptive activity during a paroxysm at the Southeast Crater was in the spring of 2000 (the most recent significant activity at the NEC was an episode of ash emission on 14-15 November 2010). There area, however, other known cases of contemporaneous (or nearly so) strong activity at more than one of Etna's summit craters, the most notable example being the series of paroxysmal events at the Voragine, the BN and the Southeast Crater on the afternoon and evening of 4 September 1999; also the nearly contemporaneous activity at BN, Voragine and NSEC on 27-28 February 2013 is to be remembered.

Fig. 11. Temporal variation of the volcanic tremor amplitude recorded at the station close to the Bocca Nuova (EBCN).